This study investigates the possibility of a sensitive period for the acquisition of lexical and collocational knowledge in a second language. The participants were ten adult native speakers of English and 38 very advanced adult non-native speakers with a range of ages of onset of learning. They performed a battery of tests, comprising a standard word-association test, an oral production task, and a range of written lexical tasks (developed for this study) focusing on the use of core vocabulary and multi-word units. The results are taken to support the existence of a sensitive period for lexical acquisition in a second language, and the overall tendency to similarity in scores obtained by NNSs with AOs of 7&#8211;12 and 13&#43; appears to indicate that this sensitive period closes around the age of six.